Pulverizers are well known for the reduction of the particle size of solid fuel to allow for combustion of the solid fuel in a furnace. A pulverizer employs some combination of impact, attrition and crushing to reduce a solid fuel to a particular particle size. Several types of pulverizer mills can be employed for the pulverization of the solid fuel, for example, coal, to a particulate size appropriate for firing in a furnace. These mills can include ball-tube mills, impact mills, attrition mills, ball race mills, and ring roll or bowl mills. Most typically, however, bowl mills with integral classification equipment are employed for the pulverization of the solid fuel to allow for the transport, drying and direct firing of the pulverized fuel entrained in an air stream.
Bowl mills have a grinding ring carried by a rotating bowl. Fixed position rollers are mounted on roller journal assemblies such that the roll face of the rollers are approximately parallel to the inside surface of the grinding ring and define a very small gap therebetween. Pressure for grinding is applied through springs or hydraulic cylinders on the roller journal to crush solid fuel caught between the roll face of the roller and the grinding ring.
An air stream is typically utilized for drying, classification, and transport of the solid fuel through the pulverizer. The air stream employed is typically a portion of the combustion air referred to as the primary air. The primary air is combustion air first directed through a preheater whereby the combustion air is heated with energy recovered from the flue gas of the furnace. A portion of the primary air is then ducted to the pulverizers. In a bowl mill, the primary air is drawn through beneath the bowl of the bowl mill and up past the roller journal assemblies to collect the pulverized solid fuel. The small particles of solid fuel become entrained in the primary air. The air stream containing the solid fuel then passes through a classifier into the outlet of the pulverizer. After passing through the exhauster, the pulverized fuel can be stored, or more typically, is transported to the furnace by the air stream for direct firing.
For example, U.S. Pat. No. 4,706,900 entitled “Retrofitable Coiled Spring System,” which issued on Nov. 17, 1987 and which is assigned to the same assignee as the present invention illustrates a prior art form of bowl mill using a coiled spring assembly for applying pressure on the roller journal to crush solid fuel caught between the roll face of the roller and the grinding ring. U.S. Pat. No. 4,706,900 discloses both the nature of the construction and the mode of operation of a bowl mill that is suitable for use for purposes of effecting the pulverization of the coal that is used to fuel a coal-fired steam generator.
The existing journal loading systems, which dictates the amount of grinding force that the grinding rolls exert on the coal as mentioned above, consist of either a spring only journal loading system or a hydraulic journal loading system. One such arrangement of mechanical spring journal loading system can be found depicted, for example, in U.S. Pat. No. 4,706,900. The spring only journal loading system consists of a spring with an integral threaded shaft that is adjusted manually which thereby changes the spring force applied to the journal. This spring force in turn would increase or decrease the load that the grinding roll imparts on the material being pulverized. Also, one such arrangement of a hydraulic journal loading system can be found depicted, for example, in U.S. Pat. No. 4,372,496. The hydraulic journal loading system incorporates a hydraulic system, which can be adjusted to change the force being applied to the journal in turn increasing or decreasing the load on the grinding roll that is pulverizing the material. The spring only method of adjusting the load on the journal does not provide a means to automatically adjust the force being applied to the journal while the mill is in operation. Further, the hydraulic journal loading system requires a large footprint external to the mill to operate and requires extensive maintenance and expertise to operate the hydraulic system.
Therefore, there remains a need for an apparatus and method for controlling and adjusting the amplitude of the load being applied to a journal assembly of a pulverizing mill. Specifically, an journal loading system is needed which is capable of being electronically controlled or adjusted that overcome the drawbacks of the hydraulic and spring only journal loading systems.